This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. The overall purpose of this study is to investigate the involvement of the brain DA system in adults with ADHD and to assess the effects of chronic treatment with the stimulant drug methylphenidate on brain DA function after a 12-14 month treatment period. The hypothesis in the current proposal that postulates compensatory decreases in DAT in ADHD is based on our pilot data in never-medicated ADHD subjects (see Section III). We will follow the research design for PET scanning as we did in IRB #353 in which we successfully enrolled and completed studies in never medicated ADHD subjects. In the current proposal, after the PET scans are completed, the ADHD subjects will return to their referring physician for a chronic treatment period of about 12-14 months and then they will be asked to return for follow-up scanning.